Cemented carbides comprised of a hard phase of a carbide and a binding phase containing cobalt, typified by WC—Co as an example, are used in various members such as cutting tools and chuck jigs because of their high hardness, excellent wear resistance and the like. Moreover, to further enhance the wear resistance and durability of such cemented carbide members, a surface of a cemented carbide member is often coated with a thin film harder than the cemented carbide, for example, a TiC thin film.
Among hard thin films publicly known, a diamond thin film has the highest hardness and excellent wear resistance. Therefore, if a cemented carbide substrate member is coated with a diamond thin film, excellent properties can be expected. However, it is conventionally considered difficult to coat a cemented carbide substrate member with a diamond thin film. The reason is that cobalt, which is the primary component of the binding phase of the cemented carbide, or any of nickel and iron, which are sometimes contained in the binding phase as required, is in general a material that can serve as a catalyst when graphite is generated, and accordingly only graphite is generated even if an attempt is made to form a diamond thin film coating on a surface of the cemented carbide substrate member on which any of these materials is present.
For methods for coating a cemented carbide member with a diamond thin film currently proposed, there is a method in which a cemented carbide member is pickled to remove cobalt on a surface of the member with an acid and then coated with a diamond thin film. However, even if the cobalt on the surface of the cemented carbide member is temporarily removed through this method, this attains little effect because cobalt inside the cemented carbide member moves toward the surface due to diffusion under a high-temperature environment for forming the diamond thin film.
Moreover, another method is also proposed in which after cobalt on a surface of a cemented carbide member is removed with an acid, the surface of the cemented carbide member is covered with a thin film of TiN or the like formed as an interlayer film to suppress the catalysis of the cobalt, and a diamond thin film is formed on this interlayer film. In the case of forming a diamond thin film with such an interlayer film interposed, problems may arise such as a matter of adhesion performance between this interlayer film and the diamond thin film, and a separation attributable to a difference in thermal expansion coefficient. Moreover, in this method, an evaporation method or a sputtering method is generally used to form the interlayer film. However, since such evaporation and sputtering methods are completely different from a CVD method for forming the diamond thin film, the process steps are increased. Further, such evaporation and sputtering methods are methods of forming a thin film in such a manner that, in theory, material particles move in a straight line, adhere to a member to be coated, and accumulate thereon. Therefore, there has been a problem that it is difficult for the interlayer film to uniformly adhere to a member having a special geometry, such as a member with projections and depressions or a member in a complex shape.
In addition, Japanese Patent Application Unexamined Publication No. H7-305170 proposes a cemented carbide member coated with a hard film in which a surface of a WC-group cemented carbide is coated with a 0.2 μm to 10 μm thick film of a metal whose carbide has standard energy of formation that becomes a negative value within a specific range at a temperature ranging from ambient temperature to 1500° C., and is then coated with a diamond thin film. The technique disclosed in this Japanese Patent Application Unexamined Publication No. H7-305170 is that the metal film formed on the surface of the cemented carbide is made to react with C during diamond deposition and is thus converted into carbide, whereby a coating of the diamond thin film with high adhesion strength is accomplished. The metal film formed on the surface of the cemented carbide is formed by a vacuum evaporation method, plating method, or sputtering method. Accordingly, the work steps and time are increased because these methods are completely different from a CVD method for forming the diamond thin film. Additionally, in the case of using such a vacuum evaporation method or sputtering method in coating, as described above, it is difficult for the metal film to uniformly adhere to a member having a special geometry, such as a member with protrusions and depressions or a member in a complex shape. In the case of the plating method, usable metals are limited. For example, a coating of a Si film could not be accomplished by the plating method.
Furthermore, Japanese Patent Application Unexamined Publication No. 2000-178736 proposes a member including a base material and a diamond-like carbon film formed thereon, wherein the member has an intermediate layer of any one of silicides, silicarbides and the like of the group 5A elements and the group 6A elements in the periodic table. The technique disclosed in this Japanese Patent Application Unexamined Publication No. 2000-178736 is basically for forming the intermediate layer. Therefore, problems may arise such as a matter of adhesion performance between this intermediate layer and the diamond thin film and a separation attributable to a difference in thermal expansion coefficient.
Patent document 1: Japanese Patent Application Unexamined Publication No. H7-305170
Patent document 2: Japanese Patent Application Unexamined Publication No. 2000-178736